The present invention relates generally to thermal response testing (TRT) for geothermal-based heating, ventilation, and air conditioning (HVAC) systems.
It is widely recognized that geothermal-heat-exchange-based HVAC systems offer operational cost advantages over conventional system alternatives. Because, however, it may be costly to construct the geothermal heat exchanger (GHEX), i.e. a closed-loop piping system constructed beneath the surface of the earth to transfer heat between a fluid and the earth surrounding the piping system (also known as a borehole heat exchanger, energy field, well field, loop field, or geo-field), it is crucial to correctly size the GHEX to maximize long-term financial performance for a facility.
Several methods and commercially available tools exist to determine the dimensions of a GHEX. The quality of the results for such methods and tools is strongly correlated to the accuracy of the input information. Oftentimes this information, especially thermo-physical earth properties, is merely assumed or estimated because of the cost and complexity of performing accurate site-specific tests, and the high uncertainty associated with industry standard testing methods. Thus, a need exists for a device that can accurately and cost-effectively measure the actual, site-specific thermo-physical earth properties, thereby allowing geothermal-based HVAC installers to install more efficient and less expensive systems.
Prior art methods can generally be divided into two categories: analytical and numerical. Static analytical methods (also commonly referred to as the line-source method) are most typical within the industry and are widely documented by industry publications such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (2007 ASHRAE Handbook—HVAC Applications, section 32.12-32.13) as well as university publications (Gehlin 1998; Mogensen 1983; Witte et al. 2002). The analytical method involves injecting heat into a GHEX at a steady rate and calculating a formation thermal conductivity value from a one-dimensional linear regression of the recorded temperature data. Analytical methods, however, provide only one of several important thermo-physical properties and may misinform GHEX designs where time dynamic interactions between the earth and the connected HVAC system are significant. This method is also costly to perform due to steady-state power requirements and the need for human interaction and supervision.
Numerical methods, meanwhile, have been proposed by researchers (see Henk J. L. Witte 2007), but have not been developed into practice. Commercially available tests and test equipment, including those provided by Geothermal Resource Technologies Inc. (www.grti.com), generally include a fixed-speed pump, an electric heating element, temperature sensors, and a data logger. The equipment is designed to perform static tests according to the analytical method only, not numerical methods.
These prior art methods also fail to measure fluid pressure, a significant component of the overall heat transfer equation. Neglecting to measure fluid pressures may result in errors of approximately 1% when calculating total heat transfer power in a typical GHEX. Fluid pressure information, especially when coupled with variable-flow fluid pumping systems, offer GHEX designers greater knowledge of the pumping requirements for the eventual geothermal-heat-exchange-based HVAC system.
Furthermore, commercially available tests and test equipment do not provide the ability to easily measure the density of the test fluid or to filter and remove air from the fluid during the test.